a) Field of the Invention
The present invention relates to a secondary image formation type view finder which is suited for use in single-lens reflex cameras using electronic image pickup devices such as CCDs.
b) Description of the Prior Art
In these days, electronic cameras (digital cameras) are attracting attention as cameras which are to used by the next generation in place of silver salt 35 mm film cameras. In diagonal lengths, image pickup devices used in the electronic cameras are several to scores of fractions of a silver salt 35 mm film. Accordingly, a focal length of a view finder optical system must be shortened when an electronic camera is to be configured as a single-lens reflex camera. Further, it is necessary to dispose an image erecting optical system between a primary imaging surface on a side of a view finder and an eyepiece optical system. Furthermore, an effective sectional area must be large in the image erecting optical system since a pupil diameter and a location of the view finder are constant regardless of a focal length of the eyepiece optical system and it is necessary to reserve a sufficiently wide field angle for upgraded cameras.
When a prism is disposed to fold an optical path of the view finder optical system so that a space to be occupied by the view finder optical system is reduced for meeting the requirements described above, the optical path is prolonged and a heavy burden is imposed on a power distribution in the eyepiece optical system, thereby aggravating aberrations which are related to image legibleness.
The burden to be imposed on the eyepiece optical system can be lessened by adopting a secondary image formation type optical system which images an image formed by a photographic optical system once in the view finder optical system as described above and reimages it with a relay lens system.
When a splitting mirror is to be used for forming a secondary image, it is desirable to select a half mirror which does not require so large a space, or more desirable to use a prism type half mirror in particular which has a small folding angle. Further, a secondary image formation type optical system uses, for light measurement, a half mirror which takes out a portion of a light bundle from the photographic optical system or the viewfinder optical system and leads it to a light measuring element.
As conventional examples of the optical system described above, there are known optical systems disclosed by Japanese Patents Kokai Publication No. Sho 63-259526, Kokai Publication No. Hei 1-185622, Kokai Publication No. Hei 1-222214 and so on.
When an object is to be photographed with a camera in an outdoor environment, the object is illuminated mostly by sunlight. Though the light from the sun is a random natural light, rays reflected by the object may be partially polarized. Accordingly, certain kinds of objects cause light bundles which are mixed with polarized light bundles to be incident onto electronic cameras.
Since half mirrors have polarization characteristics, on the other hand, their reflectance and transmittance are varied when light bundles which are polarized to certain degrees are incident on the half mirrors.
Conventional optical systems are unusable for accurate light measurements since polarization is not taken into consideration for designing these optical systems and partial polarization at different degrees varies rays which are incident on light measurement systems thereof.